Week-5-Exposure assessment and models

8/7/2019 Week-5-Exposure assessment and models

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Environmental Health RiskAssessment

PHC6311

Alok Deoraj, Ph.D.HLSII590, extension: 7-7793Email: [email protected]

mailto:[email protected]

mailto:[email protected]



• Exposure Assessment and Models• Aggregate and cumulative risk

• Risk at workplace (read the chapter-9Page 187-201, 213 last paragraph-219

and 226-234)•

•

Week 5



Learning Objectives

• Demonstrate the concept of exposure from various pathways(aggregate exposure)

• Describe different exposure models, e.g., air, water, land• Identify rationale for probabilistic assessments and statistics needed to

understand the process• Describe the uncertainty and variability in exposure assessment• Explore methods for assessing risks from multiple chemicals (and

cumulative)• Understand the scope, magnitude, distribution and temporal changes in

the major safety and health hazards workers face on the job

• Become familiar with institutions (OSHA and NIOSH) to controlworkplace hazards• Appreciate the complex interplay of science and policy involved in

controlling occupational risks•



Uses of Exposure Assessment inRisk Assessment

• Used to estimate internal dose which, with dose response data(usually in animals), is used to estimate risk.

• For risk-based regulations, provides the link to emissions (pointsource, consumer products, area sources).• Evaluation of efficacy of cleanup (risk to most exposed

subgroup).

Hazard IdentificationDose-response assessmentExposure AssessmentRisk CharacterizationRisk Communication



• What is Exposure? Dose?• General Equations for Estimating Exposure• Types/Sources of Information on Contact

Rates• Measuring vs Modeling Exposure Pt

Concentrations• Probabilistic Exposure Assessment

Important Attributes of Exposure



Why assess exposure?

• Determine factors that put segments of thepopulation at higher risk to chemical toxicity

• Help establish dose-response relationships inthe “real world”

• Hazard = Toxicity x Exposure



Use Exposure Assessment for Status and Trends

• Determine exposure at a particular place andtime as well as trends over time.

• Provide a profile of a population or apopulation segment.

• Establish effectiveness of risk mitigationstrategy (regulations).



The Importance of an AccurateExposure Assessment

• Estimated risks are based on the results.• Over-estimation of risks can lead to

unnecessarily costly cleanup.• Under-estimation can result in health risk on

human population and ecosystem.



What is Exposure?



1.Different Kinds of Doses .2.

3.Range of Exposure .• Central Tendency• High End•

3. Quantifying Exposure• Point of Contact

Measurement• Scenario Evaluation • Reconstruction

Considerations for Exposure Assessment



Exposure Scenarios

• Actual Exposures

• Reasonably Foreseeable Exposures• Hypothetical Exposures• What-If Exposures

• Counter-Factual Exposures•



Typical Exposure Scenarios

• Case I: Airborne dust/vapor • Case II: Soil• Case III: Groundwater

• Case IV: Sediment• Case V: Foods



From:Paustenbach, DJ. (2000)The practice of exposureassessment: a state-of-

the-art review. J Toxicol Env Health , 3:179-291

Illustration of Exposure Pathways



Models for EstimatingConcentration

– Atmospheric Models– Surface-Water Models– Groundwater Models– Food-Chain Models– Multimedia Models

• Distinguishing Features from a Risk AssessorsPerspective

– Input Data Requirements– Extent of Model Validation

• All models are simplified or idealized representationsof physical and/or biological processes that may betoo complicated or too poorly understood toexpress in any other way.



Morning CO Exposure Profile of an EPA Employee



Personal Exposure to Benzene



Three elements of exposureassessment

• Transportation, transformation and fateprocesses

– Before it meets up with people• Exposures

– As it meets up with people• Physiologically based pharmacokinetics

(PBPK)– What goes on In people



Exposure Elements

SOURCE/STRESSORFORMATION

DOSE

EFFECT

AcuteChronic

DispersionKineticsThermodynamicsSpatial variabilityDistributionMeteorology

Air Water DietSoil and dust

Groundwater

TargetAbsorbedApplied

ChemicalMicrobial

TRANSPORT/TRANSFORMATION

ENVIRONMENTALCHARACTERIZATION

EXPOSURE

PathwayDurationFrequencyMagnitude

Statistical profileReference populationSusceptible individualSusceptible subpopulationsPopulation distributions

• Individual• Community• Population

Transport, Transformation,and Fate Process Models

ExposureModels

PBPKModels

ACTIVITYPATTTERN



Issues in Dose and Response

Blood lead l evels

Time (Days)



Time-Weighted Average

• TWA• Total dose divided by time period of dosing• This is what is used for toxicology assumption



Exposure Assessmet_EPA

• The process of measuring or estimating the intensity, frequency,and duration of human contact with agents currently presentin the environment or the hypothetical contact that mightarise from their release in the environment.

• The EPA Guidelines for Estimating Exposure (U.S. EPA, 1986a)

defines exposure as the contact with a chemical or physicalagent.• --The magnitude of this contact is determined by measuring or

estimating the amount of an agent available at the exchangeboundaries during some specified time.

• --Once the agent is absorbed through these boundaries, theamount crossing the boundary becomes the absorbed dose.

• The primary purpose of an exposure assessment is usually toestimate the real-world dose (exposure) value to use in adose-response relationship.



Methods of exposureassessment vary with needs

• A highly sophisticated exposure assessment may beneeded if the objective is to ensure that noindividual is overexposed to a dangeroussubstance

• Only screening exposure assessment may beneeded as an approximate estimate of exposurefor priority setting

• Exposure should be assessed so that it can berelated to dose (and possible health effect) withsufficient accuracy and precision to meet research,regulatory, or exposure control objectives



Each assessment strategypresents different issues of

relevance

• Data

• Sample type personal/area• Chem. Specific• Time specific

• Location specific• Activity specific



Other fate/ Transport Factors

• Persistence• Microorganisms• Light

• Moisture• pH• Temperature• Half-life



Other Factors in ExposureAssessment• Duration and frequency of exposure must be considered in anexposure assessment. In terms of duration , exposures may

be acute (one-time), chronic (repeated, for a substantialfraction of the lifespan (example: 10%) of a lifetime). Exceptfor acute exposures, there are no standardized quantitativedefinitions of these terms. Frequency of exposure is alsoimportant-exposure may be continuous (daily) or intermittent

(less than daily, with no standardized, quantitative definition).•

• Finally, it is important to know, for exposures of limited duration,the time in life during which exposure took place . For ateratogenic agent, for example, it is essential to knowwhether exposure took place or could take place during thesubject’s pregnancy.



Aggregate Exposure

• Sum total of exposure to a chemical via ALLroutes of exposure and in all media

• Concentration times duration•

• E.g., DDT:– 6 to 10 sources (fruits and veggies)– Three routes (air, food, water)



Lifetime Average Daily Dose (ADD)

• 70 year old person• Has eaten lettuce

since age 4 (14,000

kg)•

•

• Bioavailability• 4 mg Aldrin per kg

lettuce

LT BW

BDIRC LADD

⋅

⋅⋅⋅

=



Examples of direct monitoring

• The best-known example of the directmeasurement of exposure is the radiationdosimeter

• CO Assessment by EPA• Individuals randomly selected; Interviewed by

telephone and screened to obtain smaller stratified population

• Stratified by CO Exposure Risk Factors:Smoking, commute time, other

• Personal CO Monitor used for several days• Urban CO Exposure profile established•



Indirect Monitoring

• Site selection influences results (e.g. spacialand temporal variation)

• Must incorporate pattern analysis for exposure estimates job classification

• Human activity patterns•

•



Spacial Variation

• Random Sampling: Monitor locations selectedin a random manner so that it is notpossible to predict location of any samplingpoint based on the location of others

• Systematic Sampling; laying out a grid• Initial point selected randomly• Assures uniform sampling across areas• More complex statistically



Spatial Variability in Pollutant Air Concentration (ug/m 3) Downwind of an

Industrial Emission Source



Temporal Variation

• Sequential measurements at one site• Temporal correlations must be accounted for;

if ignored, mean and confidence intervalunderestimated

• E.g. concentration of a contaminant in anaquifer measured at a given well on oneday depends on the concentration on theprevious day.



Temporal Variability in Pollutant Air Concentration (ug/m 3) Downwind of an

Industrial Emission Source



Models for Exposure Assessment

• A model is a mathematical expressionrepresenting a simplified version of exposure processes.

• Provides a means by which diverse data onrelevant factors can be combined to predictlevels of human or environmental exposure

• Modeling is an iterative process of input andrefinement

• Large range of models of different complexity,

from back of the envelope to complexcomputer simulations (EPA-Air PollutionModels)



Types of Models

• Gaussian Plume Model- plume from anemission source spreads laterally andvertically, ascending to a Gaussiandistribution

• Trajectory Models-compute the trajectory thata pollutant might follow.• Puff Transport Models- rapid, short-duration

emissions

• Compartmental Models

P ll i F d T



Pollution Fate and TransportModels

• Objective- determine the averageconcentration of a pollutant in time for apopulation by one or more exposurepathways

• Pollutant may be a chemical or biologicalagent

• Time may range from seconds to years• Exposure pathways-standard•

V i bl i diff t ll t t



Variables in different pollutanttransport and fate models

• Environmental transport media (air, surface or groundwater, biota)• Geographic scale (global, national, regional, local)• Pollutant source characteristics (continuous or

instantaneous release, industrial, residential or commercial, and point or area sources

• Risk agents (e.g., a specific compound or class of related subjects)

• Receptor populations (normalhumans/animals/plants/MO, highly exposed,susceptible)

• Exposure routes (typical or unusual e.g. breast milk)• Time Frame



Atmospheric Models

• Focus on pollution transport, diffusion, and deposition•

• Transport - movement of suspended of pollutant through theatmosphere

• Diffusion - microspread and dilution of individual particles andmolecules

• Deposition - transfer to ground/water or vegetation (wet or dry)• Many variables influence transport/diffusion/deposition• Atmospheric stability (resist or enhance vertical motion of the

air)• Temperature inversion• Industrial emissions dispersion a function of: stack velocity,

temperature not atmospheric stability, and stack height•

• Model Outputs• Atmospheric concentrations• Wet and dry deposition rates



Atmospheric Dispersion Model

•

Example of a validated model



Case Air pollution

• Location of Exposed Persons• On-site• Off-site• Air Concentration• Peak concentration (1 and 24 hours)• Annual average concentration



Air Contaminants (When?)

• Exposure Duration• Constant exposure• Routine (but non-continuous)• Sporadic



Air Contaminants (How much?)

• Determine• Concentration at point of exposure• Breathing rate per body weight• Absorbed dose (uptake)




• Estimating Concentration• Direct measurements• Indirect measurements• Published emission rates• Mathematical models for emissions• Dispersion models

Air Contaminants from Soil



Air Contaminants from Soil(How Much?)

• Mathematical Models• Farmer’s Model• Jury’s Behavior Assessment Model (BAM)• Fugitive dust model• Particulate emission models



Air Contaminants from soil

• Factors Affecting Vapor Flux• Physical properties of chemical• Vapor pressure• Solubility• Saturation Vapor Density• Adsorption Tendencies• Molecular Weight



Other Factors

• Properties of Soil Matrix• Bulk Density• Porosity• Moisture Content• Organic Carbon Fraction



Other Factors (Vapor from Soil)

• Environmental Factors• Humidity• Temperature• Barometric Pressure• Precipitation• Wind Speed




• Dose= Concentration (mg/m^3) * Ventilation(m^3/hr) * bioavailability (%)



Contaminated Soil

• Residential (children/adults)• Industrial (adults)• Parks/Recreation (children/adults)• Sediments due to Runoff (fishermen/fish)• Wildlife (grazing animals)



Contaminated Soil (How?)

• Residential (children eat soil or dust)• Industrial (dermal contact)• Agricultural (food)• Parks/recreation (ingestion/dermal contact)• Wildlife (soil ingestion/forage)•



Dermal Exposure Parameters

• Concentration in soil, dust, or water • Soil/dust deposition rate from the air • Direct soil contact• Skin permeability rate• Area of exposed skin• Body weight



State-of-the-Art Issues

• Consider using site/sub-population exposureparameters• Soil ingestion• Water ingestion• Bioavailability• Greater use of Monte Carlo analysis• Better presentation of uncertainty



Empirical Data

• Direct measurement• Usually measures applied dose• A variety of methods and equipment have

been developed



Biological Monitoring

• Body burden levels or biomarkers• Concentration of chemical in tissues or sera– Usually not the tissue of concern– Need to understand internal dose relationship

• Concentration of the chemical’s metabolites• Biological response chemicals• Chemical or metabolites bound to target

molecules



Modeling Exposure

• “Exposure Scenarios”• Recreating past doses• Predicting future doses• Two major components

– Chemical concentrations (including timetrends)

– Population characterizations

Important goals for the Improvement



Important goals for the Improvementof Exposure Information

• Collect data over time (Establish a “baseline”, and followtrends)

• Establish standard methods and protocols (Use standard

methods and protocols, and apply consistentrequirements for quality control/quality assurance)• Develop statistically representative sampling data (Allow

extrapolation beyond the individual study)• Collect more measurements of exposure• (For developing, validating, and refining human exposure

models)• Support epidemiologic studies





• Collect data over appropriate time frames(Support epidemiologic studies, allowevaluation or prediction of acute and

subchronic, as well as chronic effects• Characterize total human exposures (Allow

evaluation of total exposures to individual,multiple polluntants)

• Allow source apportionment or identificationof key sources of exposure





• Characterize exposures to pollutant mixtures (For individual routes of exposure)

• Identify high-risk groups (Identify biologically susceptiblesubpopulations and subgroups receiving exposures atupper tail of exposure distribution, or “high-end”exposures

• Address environmental inequities• Identify regional, ethnic, or socioeconomic

subpopulations likely to receive “high-end” exposures• Develop distributions of exposure (Allow characterization

of variability and uncertainty in exposure parameters,estimates, and measurements

http://www.epa.gov/ncea/pdfs/efh/front.pdf



Exposure Factors Handbook

• Drinking water consumption rates

• Breast milk consumptionrates

• Consumption rates of foods

• Soil ingestion rates• Breathing rates

• Body surface areas• Body weights• Shower times, intensities,

temperatures• Animal exposures

– Domestic– Wildlife


http://www.epa.gov/ncea/pdfs/toc2-37.pdf

http://www.epa.gov/ncea/pdfs/toc2-37.pdf




Standard Assumptions

– 70 yr life span– 70 kg Body weight– 2 L/day water consumption– 200 mg/day

Variability?GeographicCultural

Variability versus central tendencies



Dermal exposure

• Cutaneous permeability• Dermal bioavailability• Skin surface area• Soil loading on the skin



Deterministic Model

• Point Values for Each Variable• Some Typical or Average Values• Some High End or Upperbound Values



Monte Carlo simulation was named after the city in Monaco (famous for its casino) where games of chance (e.g., roulette) involve repetitiveevents with known probabilities. Although there were a number of isolated and undeveloped applications of Monte Carlo simulationprinciples at earlier dates, modern application of Monte Carlo methodsdate from the 1940s during work on the atomic bomb. MathematicianStanislaw Ulam is credited with recognizing how computers couldmake Monte Carlo simulation of complex systems feasible

History of the Monte Carlo Method



http://hyperphysics.phy-astr.gsu.edu/hbase/math/dice.html

The probability of getting a given value for the total on the dice maybe calculated by taking the total number of ways that value can beproduced and dividing it by the total number of distinguishableoutcomes. So the probability of a 7 on the dice is 1/6 because itcan be produced in 6 ways out of a total of 36 possible outcomes.

k k f h l l





Skin uptake of a chemical in soil

• Uptake = C × A × r × B• C in mg material per kg soil• A in cm 2

• r in mg / cm 2

• B is unitless (bioavailability)

M C l A l i



Monte Carlo Analysis

• Uptake = C × A × r × B• What if we know distributions of C, A, and r,

and uncertainty surrounding B!• MEI (maximally exposed individual)• 95% worst case for each?• 1 - (1-0.95) 4 = 99.9994 case?

M C l A l i




• A taste:• C = lognormal (12 mg / kg, 3 mg / kg)• A = 500 cm 2

• r = uniform (0.015 kg / cm 2,0.025 kg / cm 2)• B = lognormal (0.75, 0.02)• Mean Uptake = 70 mg• Upper 95%? = 180 mg / kg

M C l A l i




• 95% upper CI?• C = lognormal (12 mg / kg, 3)• A = 500 cm 2

• r = uniform (0.015 kg / cm 2,0.025 kg / cm 2)• B = lognormal (0.75, 0.02)• Uptake = 70 mg



•

Risk at workplace•

•

• (Read chapter-9: Page 187-201, 213 last• paragraph-219 and 226-234)

•

•

Week-5-Exposure assessment and models

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